Question: Solve for $r$ : $-23 = r - 12$
Solution: Add $12$ to both sides: $ \begin{eqnarray} \\ \hphantom{-23 {+ 12}} &\hphantom{=}& \hphantom{r} \\ -23 &=& r - 12 \\ \\ {+12} && {+12} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ -23 {+ 12} &=& r \\ \end{eqnarray} $ Simplify. $ r = -11$